As integrated circuits continue to shrink in size, the processes by which they are formed are increasingly limited by fundamental physical laws. For example, in forming structures that are less than about a quarter micron in length, or in other words less than about 250 nanometers long, such as gate structures in a metal oxide semiconductor device, the ability of the radiation used to pattern the structure during the photolithography process is seriously challenged. Photolithography processes typically use ultraviolet radiation with a wavelength of about 248 nanometers to expose the photoresist used to pattern the structures. Unfortunately, a beam of light beam with a wavelength of 248 nanometers has difficulty in resolving the closely spaced features in a masking pattern that is not appreciably greater than the wavelength of the light beam. Thus, the processes used to form integrated circuits must necessarily change as even smaller device features, such as 100 nanometer gate lengths, are desired.
One method of forming devices with smaller features is to use electromagnetic radiation with smaller wavelengths during the photolithography process. For example, electromagnetic radiation with a wavelength of 193 nanometers provides the ability to pattern features that are about twenty percent smaller than those patterned; with electromagnetic radiation having a wavelength of 248 nanometers. However, moving to steppers and other exposure tools that utilize 193 nanometer technology is still insufficient, of itself, to produce 100 nanometer features. Radiation with even shorter wavelengths, such as 157 nanometers, presents serious cost considerations and other technical challenges. Thus, other improvements to the photolithography process are required.
Some of these other improvements provide for the ability to accomplish so-called sub wavelength patterning of photoresist. By this it is meant that the techniques employed provide the ability for the electromagnetic radiation to pattern features that have dimensions that are smaller than the wavelength of the electromagnetic radiation so employed. One such technique is the use of phase shift masks.
Phase shift lithography provides control of the phase of an exposure of light beam at a target. Adjacent bright areas are formed which are preferably 180 degrees out of phase with one another. Dark regions are produced between the bright areas by destructive interference between the radiation phases. One problem with phase shift masks is that they are typically selected to be compatible with the particular photoresist material being patterned. In order to be effective, the photoresist material being patterned must be closely matched to the phase shift mask being used. However, it is desirable to use a wider variety of photoresist materials to provide closer spacing and finer definition of patterned integrated circuit devices.
What is needed, therefore, is a phase shift mask with variable transmission properties that allows for the use of different photoresists.